The present invention relates to injection molding of ceramic articles and, in particular, to the production of dense, hard articles having industrial uses. Although the present invention will be discussed in regard to compositions containing silicon carbide as the ceramic material, it will be understood that other sinterable metal carbides, for example, titanium carbide, may be utilized as the ceramic material.
Silicon carbide has long been known for its hardness, strength, and excellent resistance to oxidation and corrosion. Silicon carbide has a low coefficient of expansion, good heat transfer properties, and maintains high strength at elevated temperatures. In recent years, the art of producing high-density silicon carbide materials by sintering silicon carbide powders has been developed. High-density silicon carbide materials find utility in the fabrication of components for turbines, heat exchange units, pumps, and other equipment or tools that are exposed to severe corrosion or wear, especially in operations carried out at high temperatures. The present invention relates to sinterable metal carbide compositions which may be injection molded and subsequently sintered to produce high-density products. The present invention further relates to the use of silicon carbide as the ceramic material and, in particular, to ceramic compositions containing predominantly the alpha crystalline form of silicon carbide.
Ceramic bodies or compacts may be formed by various processes, for example, by slip casting, by pressing a green body followed by sintering, or by hot pressing, and subsequently shaping or machining of the sintered body to produce the finished product. Injection molding is a process wherein a moldable composition is forced into a mold or die. Injection molding allows molding of complex shapes. The injection molding process facilitates a rapid and repeated forming of a plurality of articles having a consistent shape with close dimensional tolerance. The injection molding process minimizes the amount of shaping or machining that may be required to produce a finished article. Injection molding of the present ceramic compositions may be carried out by either the transfer method or by the direct injection method utilizing known injection molding techniques.
Many problems are encountered in injection molding of ceramic materials, especially if particulate silicon carbide is utilized as the ceramic material. The irregularly shaped particles tend to pack as pressure is applied; ceramic materials have an innate resistance to flow. The packing of such materials in pressing or molding operations frequently causes internal stresses in the pressed or molded article which lead to consequent failure of the finished product. The abrasive properties of the ceramic materials must be considered, as the material itself rapidly wears away all surfaces with which the ceramic particles come in contact. Repeated use of ceramic materials, especially those containing silicon carbide, is extremely detrimental to the mold or die surface against which it is forced. Such wear soon makes the mold less accurate, and additional finishing steps are required to obtain and maintain close tolerances in the molded articles. Thus, injection molding of ceramic materials, such as silicon carbide, has not been deemed heretofore to be economically feasible.
Co-pending application Ser. No. 864,415 filed Dec. 27, 1977 now U.S. Pat. No. 4,144,207 describes compositions and processes for injection molding ceramic compositions containing silicon carbide as the ceramic material. The present compositions are useful in the processes described in that application.
In a typical method of injection molding ceramic compositions which include silicon carbide, the compositions contain from about 65 to about 85 percent by weight ceramic material. The remainder of the composition comprises sintering aids, a thermoplastic resin material, suitably an oil or wax having a vaporizing point less than that of the thermoplastic resin material, and, for best results, a lighter, more volatile oil. The components are mixed and formed into an article by known injection molding techniques. The formed article is then baked at temperatures between about 700.degree. and about 1000.degree. C. in a nonoxidizing atmosphere to remove the resins, waxes and oils by destructive distillation in a manner to prevent the removal of the residual char carbon. The baked article is then sintered at temperatures between about 1900.degree. and about 2200.degree. C.
Up to now, it has been most difficult to obtain loadings approaching 75 percent by weight or above of the ceramic material, e.g., silicon carbide, in the composition because, when higher loadings are used, the flowability of the composition is adversely affected.